The present disclosure generally relates to anti-fouling coatings for use on components employed in combustion systems and exposed to slag, ash, and/or char.
Combustion systems, such as gasifiers and pulverized coal fired plants, are often used to convert carbon based feedstocks to synthetic gas, called “syngas.” Gasification can use a wide range of fuels including coal, petroleum coke, biomass, oil refinery bottoms (waste oil), digester sludge, and virtually anything that contains carbon and can be fed into a gasification chamber. The high temperature (generally at about 1150 to about 1,700° C.) melts the inert material and then flows to the bottom of the gasification vessel where it is cooled into a glass-like non-leachable inert slag. This slag is used primarily as aggregate in road gravel or concrete applications.
In an integrated gasification combined cycle (IGCC) facility, an air separation unit separates air into its component parts and sends the gasifier a stream of pure oxygen. The gasifier then produces the syngas from a variety of fuels. For example, coal slurry is injected with oxygen at high pressure as it is fed through injection nozzles into the gasification chamber where it can then be converted into syngas. Byproducts of the gasification process include slag, ash, and char, among others. Syngas is then cooled using either radiation or convective heat exchangers for heat recovery for energy generation and to cool the gas before it is piped through environmental control processes where pollutants and particulates are easily removed and this is called the “gas clean-up” phase of gasification. For example, the raw syngas can pass through a series of coolers to reduce the temperature and produce high-pressure steam. The recovered heat can be used to preheat clean syngas and boiler feed water. Afterwards, syngas can be cleanly burned in a combined cycle gas turbine. Combined cycle technology is composed of gas turbines, steam turbines, and their supporting infrastructure.
One of the problems in current combustion systems such as the IGCC system discussed above or in gas turbines for either aviation or land based (CMOS) is the build up of slag, ash, and/or char on internal surfaces i.e., surface fouling. As used herein, ash generally refers to the remnants of completely combusted particles of the fuel. The ash particles in the hot gas stream have not reached the melting temperature of the mineral matter originally contained in the fuel. These ash particles are typically less than 74 microns in size. The composition of the ash produced from a metals containing liquid hydrocarbonaceous fuel substantially generally includes a metal selected from the group consisting of Fe, Zn, Ni, V, Na, and mixtures thereof, and the metal sulfides and possibly oxides thereof. The term slag refers to substantially molten ash or molten ash that has solidified into glassy particles. Slag particles are remnants of completely burnt fuel particles or slurry droplets and represent the fused mineral (sand, rocks, etc.) matter of the fuel feed. The content of mineral matter, e.g. ash in a typical solid carbonaceous fuel in weight percent may be about 0.2 for petroleum coke and 20.0 for coal. Char is the de-volatilized and partially combusted solid carbonaceous fuel particles consisting mainly of ash. The remainder of the char e.g. about 2-65 wt. % comprises carbon, and little, if any, hydrogen and/or sulfur. The particles have not reached the melting temperature of the mineral matter originally contained in the solid carbonaceous fuel.
Typically, the buildup of slag, ash, and/or char on various surfaces is mechanically removed when the respective combustion system is taken out of service. In some instances, this requires an operator to physically hammer the slag from the walls. There are currently no physical barrier coatings that are designed to prevent ash deposition onto and slag reaction with surfaces in contact therewith at the high combustion temperatures found in gasification plants or potentially other combustion systems. The push to use ceramics for higher operating temperatures in fossil fuel combustion systems is dramatically limited by the reducing atmosphere present within and the deposition and chemical attack by slag byproducts contained in the combustions gases.
Accordingly, there remains a need for a barrier coating that resists chemical attack by molten slag, prevents buildup of slag and ash, and can withstand the harsh environment of combustion systems.